High-availability systems typically have multiple boards or cards arranged in parallel slots in a chassis or backplane. With this type of scheme, boards or cards may be removed from, and inserted into a live backplane at will. A board may have a large capacitance, and the backplane may have some inductance between the power supply and the board connector. Fast changes in current through a switch between the board and the backplane to charge a large capacitive load may cause a power droop or ringing on the backplane due to the fast change in current though the backplane inductance. This can result in undervoltage and overvoltage conditions in the boards, cards or chips on the backplane power supply line.
Each plug-in module usually has a local Hot Swap™ controller, ensuring that power is safely applied to that board during both rigorous hot-swap events, and steady-state conditions. The Hot Swap™ controller allows a board to be safely inserted to and removed from a live backplane. The Hot Swap™ controller must protect against large inrush currents, over-voltage and under-voltage faults, and backplane voltage transients. An example is the LTC4214 device produced by the Linear Technology Corporation, the assignee of the current subject matter.
When circuit boards are inserted into a live backplane, power supply bypass capacitors can draw a large transient current or inrush current from the power bus as they charge. The primary function of a Hot Swap™ controller is to limit this inrush current to acceptable levels, allowing an operator to insert boards quickly and easily without having to power-down the system. Without this orderly application of load current, the board and connectors could be severely damaged and the backplane voltage may be pulled down or ring.
If a current fault, such as a short-circuit condition, occurs on a board after start-up, the controller should isolate the board from the supply, ensuring that the other boards in the rack are kept operational, and that a single faulty board will not pull the backplane voltage down, causing system-wide failures. In high-availability applications, where uptime is critical, the controller should permanently disconnect the board only when the current fault is permanent. Board shutdown due to transient current faults should be avoided, but every effort should be made to keep the board safe while assessing the seriousness of an over-current fault.
A properly set inrush current profile that limits the current slew rate, i.e. the change in current over time (dI/dt), guarantees that the current in the supply inductance changes slowly enough that supply droop and ringing are not problematic for a capacitive load of any value. Hence, there is a need in circuitry to limit the slew rate of the inrush current in circuits that switch power between a backplane and a daughter board or card, to prevent glitches on the backplane power supply that can influence circuits resident on the backplane.